The adenosine A2a receptor has recently been shown to play a critical role in negatively regulating immune responses in vivo. Our laboratory has been interested in dissecting the signals that promote TCR-induced activation versus tolerance. During the course of our studies we have found that the adenosine A2a receptor is highly upregulated during the induction of T cell anergy. Based upon preliminary data we hypothesize that A2a receptor engagement on T cells promotes the induction of T cell tolerance. Using T cell clones in vitro we will define the ability of A2a receptor agonists to promote T cell tolerance even in the setting of costimulation. Using A2a knockout mice we will define the specificity of the A2a receptor in promoting tolerance and the role of endogenous adenosine in promoting tolerance. Further we will define the role of the A2a receptor on antigen presenting cells (APCs) and target tissues. Using an in vivo model of T cell mediated autoimmunity we will demonstrate the ability of A2a receptor engagement to prevent T cell mediated death by promoting anergy and antigen specific Lag-3+ T regulatory cells. Interestingly, the tumor microenvironment contains high levels of adenosine. As such, we propose that tumor-derived adenosine facilitates the induction of tumor-specific T cell tolerance. We will test this hypothesis using A2a specific antagonists and T cells from the A2a knockout mice in a well defined murine model of prostate cancer. We predict that by inhibiting A2a receptor engagement, we will be able to prevent/overcome tumor-induced tolerance and thus enhance the efficacy of tumor vaccines. We will also define the mechanism of A2a-induced tolerance. Previously, we have shown that the binding of CREB and CREM to the -180 site of the IL-2 promoter plays an important role in repressing IL-2 transcription in anergic T cells. In as much as A2a engagement leads to the generation of cAMP, we will test the hypothesis that A2a engagement mediates its inhibitory effect in part by enhancing the binding of the CREB/CREM at this site. Furthermore, based on preliminary data we will test the hypothesis that the novel cAMP activated target EPAC is a critical downstream effector. Understanding the role and mechanism by which the A2a receptor promotes T cell tolerance should provide insight in terms of devising specific clinical targets.